A silver salt photographic color copying machine is used to make a color copy from a color original. It is, of course, desired to reproduce an image having a density corresponding to the original in good faith. A key is how to reproduce a density and other factors in as close agreement with the original as possible.
A typical prior art copying method is designed such that prior to actual copying, photometric measurement is carried out on the surface of the original to read various types of density necessary to reproduce a proper image, and an optimum exposure amount for copying is determined from such read data.
The types of density read include a density of the original's background (non-image portion), maximum density, minimum density, average density, contrast, an intermediate or weighted average between maximum and minimum densities, a peak density in a histogram, an average of densities appearing at a certain frequency or higher in a histogram, or the like. The thus read density value about the original is utilized in reproducing an image of the original. Several typical methods are described below.
(1) Japanese Patent Application Kokai No. 60-10267 discloses a method comprising previously storing a nonlinear characteristic curve indicative of the relationship of original density to illuminant quantity (exposure) in an exposure computing means, selecting three original densities corresponding to true black, true white and an intermediate density on the basis of the difference in density of the original, and correcting the illuminant quantity each time when the original is copied.
(2) Japanese Patent Application Kokai No. 59-13232 discloses to tailor copying process conditions such that a copy of thinner color may be produced from an original of a deeper color or with a deeper color background and vice verse.
(3) Japanese Patent Application Kokai No. 59-15264 discloses to classify the composition of an original in terms of maximum and minimum densities appearing at a certain frequency or higher in a density histogram and to reproduce an image in accordance with the classification.
(4) Japanese Patent Application Kokai Nos. 54-92742 and 58-113949 disclose an electrophotographic apparatus wherein an image is reproduced by controlling a bias potential in terms of the type of original and modifying the .gamma. or gradient of a copy image.
(5) Japanese Patent Application Kokai No. 60-178469 discloses a method for reproducing a copy image by manually setting previously classified reproducible types of original, applying a basic characteristic curve corresponding to a particular type of original to be copied, variably controlling exposure or developing conditions for each type of original, and varying the contrast for each type of original.
(6) Japanese Patent Application Kokai No. 60-22135 discloses to reproduce an image by previously determining classes of an original on the basis of the contrast between an original background and a character pattern and allocating an original to be copied to any one of the classes.
(7) Japanese Patent Application Kokai Nos. 60-112068 and 58-184160 disclose to utilize a minimum density, maximum density or intermediate therebetween of an original to determine an exposure for copying such that an image is reproduced in as close agreement with the original as possible.
It is also known to select a density of the highest frequency in a density histogram of an original or a subject to be copied as an image density. An exposure is computed using the selected density.
When it is desired to reproduce an image having a density with fidelity to an original, there are many factors to be considered, including not only the above-mentioned photometrically measured density and exposure, but also problems occurring on the copying and developing apparatus sides, for example, a variation and a performance lowering due to a change with time of a copying optical system, a variation in characteristics and a lowering in characteristics due to a change with time of a photosensitive material and developing material which are image-forming materials, and a variation in developing conditions.
In general, originals which are contemplated to be copied in these prior art methods bear a two-gradation image (black-and-white image) or an image with a lesser number of gradations as in the case of originals with a black-and-white pattern, character originals, and illustrated originals. In copying, it is only required to reproduce a white area or background at proper white density and characters and black-and-white patterns at proper blackness.
Although the prior art methods are effective in reproducing a two-gradation image or an image with a lesser number of gradations, they are not necessarily effective in reproducing a multi-gradation image as typified by a color image. This is because the multi-gradation image to be reproduced herein has a complex profile of gradation and a slight difference in density between gradations.
For example, one prior art method for reproducing an image utilizes the information about an original including maximum and minimum densities. An exposure is determined from the maximum density corresponding to the density of characters and the minimum density corresponding to the density of background areas. When applied to a multi-gradation color image, this prior art method fails to properly reproduce most areas of intermediate density. As to the other prior art method utilizing a peak density in a histogram, a background area is moderately reproduced so as to finish the remaining image area properly, or character areas are reproduced to a proper density so as to finish the background area to adequate whiteness. Since this method relies on the principle that the character density has an approximately constant difference to the background density irrespective of the identity of original, it is not applicable to multi-gradation images.
Selection of density values by these prior art methods after reading the density of an original is effective insofar as black-and-white images are to be reproduced. Such selection is inadequate when it is desired to reproduce multi-gradation images such as color originals with fidelity.
The above-mentioned problems occurring on the copying and developing apparatus sides occur to different extents even among apparatus of the same type. To avoid any influence by such phenomenon, each apparatus must be given a particular set of copying conditions. In addition, a provision is also necessary for accommodating a change of .gamma. characteristic of a copying or photosensitive material.
The prior art method utilizing the information of minimum density also has a problem. An original to be copied often has a smaller area than the surface of a table of the apparatus on which the original is rested or the area of photometric measurement. Sometimes, the original is not rested on the table in place. In such cases, light irradiated for scanning of the surface of the original can be transmitted by the area of the table outside the original, reflected by a white surface of the retainer plate, and then directly passed to photometric measuring means. The measurement of image density is thus largely affected, resulting in a markedly lowered accuracy of measurement.
The other prior art method utilizing the information of maximum density raises a problem when it is desired to make a copy from an original in the form of a voluminous book. The book is rested on the table with necessary pages open to the table surface. Within the region which would otherwise essentially belong to the region of the original, large gaps are defined between the pages and outside the perimeter of the book. Upon copying, scanning light irradiated toward the original is transmitted by those areas of the table corresponding to the gaps. A part where no or little reflected light is available is thus created in the original's region where reflection of light is predicted over its entire area. When such a phenomenon occurs, a density having an extremely high value is photometrically measured on the original, causing the precision of measurement to lower. This phenomenon often occurs when three-dimensional articles such as packages and commercial items are to be copied.
Most prior art methods have not taken into account these problems. A few methods give consideration in these respects, but are still unsuccessful.